How to add support for SAM D51/Cortex M4?

Hello,
my name is Karl-Heinz and I discovered MySensor a few days ago. I am very impressed and would like to use it for all my sensors.
I have 8bit and 32bit MCUs.
My latest MCU is an SAM D51 (Adafruit Itsybitsy M4).
I can use it when I make a minor modification to MySensors/hal/architecture/SAMD/MyHwSAMD.cpp.
When I try to compile Arduino IDE throws an error, that ADC is not defined in this context -> uint16_t hwCPUVoltage()
I have not much experience with programming. Therefore I commented all code lines in uint16_t hwCPUVoltage() and added hardcoded "return 3.3;"
Then I can compile at least what I tested so far for the SAM D51.

Of course this is not a solution. Therefore I would like to ask what is the proper way to modify MySensors for use with the Cortex M4 processors.